This invention relates to a display memory system and, more particularly, to a display memory system wherein the write display memory rate and read display memory update rate are non-integer multiples of each other, alternately known as a Gerbil Wheel Memory (GWM).
Prior art display memories cannot have a read rate and a write rate which are non-integer multiples of each other. For example, it may be advantageous to write a memory buffer with video data at an update rate of 20 Hz, while simultaneously reading video data from the same buffer at an update rate of 66 Hz. A conventional PING/PONG video memory in which the input and output update rates are integer multiples of one another (for example having a 30 Hz input rate and a 60 Hz output rate) is not a feasible solution.
A conventional PING/PONG memory configuration utilizes two full frames of dual port random access memory (RAM). The frame being updated is called write display memory. The frame being displayed is called read display memory. The write display memory device writes to one frame while a read display memory device simultaneously and synchronously reads from the other. When both devices have finished their respective memory accesses, which will occur at the same time, the read display memory device then reads from the frame previously written to by the write display memory device, and the write display memory device begins writing to the frame previously read by the read display memory device. This configuration is adequate only when the input writing rate and output reading rate are integer multiples of one another. If the frame rates are not integer multiples of one another, the update rate of the read display memory device will approach the output rate of the write display memory device, resulting in loss of data or overwriting of data.
Prior art solutions have not addressed the particular problem of input and output rates that are not equal or not equal multiples of each other. Current display memory systems demand that the write update rate be slaved to the input video source or to the rate the display requires. As a result, most, if not all current systems, use a PING/PONG configuration.